Unlike interferometers used to measure a Doppler frequency shift directly, the practical use of the Sagnac is not restricted to very high velocities. It finds particular advantage in the measurement of high frequency oscillatory velocities, where the direct response to velocity effectively discriminates against unwanted low frequency components. To optimize the sensitivity of the interferometer, we have introduced a .pi./2 phase bias between the two beams, using a passive technique based on control of the birefringence of the fibre loop.
Fibre optic techniques in laser velocimetry are known (Jackson, D. A., Jones, J. D. C. 1986 Fibre Optic Sensors. Optica Acta 33, 1469, Jackson, D. A., Jones, J. D. C. 1986 Extrinsic fibre optic sensors for remote measurement: parts one and two. Opt. and Laser Tech. 18, 243, 299). Many commercial instruments are now available which use fibres to form flexible waveguides between a source/detector module and a remote probe. It is thus appropriate to consider further applications of fibre optics in which their properties are exploited to facilitate optical signal processing of the Doppler signal. In particular, the use of fibres and fibre components allows the implementation of interferometer configurations which are impractical using conventional optics.